I came across this the other day when I was looking up how to do matrices for an assignment. Pretty good, not too long and quite comprehensive - it shows you the Latex code by example which I found useful. Hopefully may be of use to someone!
I also find detexify.kirelabs.org/classify.html to be a good place to quickly look up a symbol that you can't remember the name or code for. So long as you can draw a semi-reasonable version of it with your mouse you can get the latex code easily.